Material blenders are known which include a vertically oriented vessel with a centrally mounted lift column for recirculating material within the vessel. Typical examples of such blenders are shown, for example, in U.S. Pat. Nos. 3,276,753; 3,642,178; and 4,194,845.
Gravity type blenders include a vertically oriented vessel with a plurality of downcomers each having inlets at various levels in the vessel. Material in the upper part of the vessel enters the downcomers into a receiving bin or hopper so that material from various levels in the vessel are mixed. In some instances, a material recirculation system is provided. Typical examples of such blenders are shown for example in U.S. Pat. Nos. 3,158,362; 3,216,629; 3,421,739 and 4,068,828.
Bottom fill blenders include a central lift column for blending solid particulate material such as plastic pellets. Such apparatus are generally shown in U.S. Pat. No. 4,569,596 and U.S. patent application Ser. No. 680,213 filed Dec. 10, 1984, now U.S. Pat. No. 4,573,800, both assigned to the assignee of the present invention. In this type of system, the material to be blended is pneumatically conveyed from a source of material to the bottom of the blender and the energy utilized for conveying the material to the blender is used to lift the material up the central lift column entraining material already in the vessel lifting the same to the top of the vessel and, thereby, blending the material. Top fill techniques are also known in the art.
U.S. Pat. Nos. 4,068,828 and 3,592,446 disclose systems capable of continuous blending of particulate materials with particles being introduced at one location within a vessel and withdrawn at another location within the vessel at the same time.